1. Field of the Invention
This invention relates to gas turbine engines variable bypasses and particle removal and more particularly to variable bypass flow splitters integral with surge bleed doors that pivot into the engine flowpath in a transition section between the booster and core engine compressor sections.
2. Description of Related Art
It is well known in the gas turbine engine field to provide variable bleed valves (VBVs) having doors that open to provide a bleed path to bleed off compressed air between the booster and core engine compressor of gas turbine engines. Aircraft fan jet gas turbine engines and marine and industrial derivatives of such engines have employed various forms of curved flowpaths and VBV bleed doors that are retracted into the flowpath casing so as to form an entrance to a bleed duct that bleeds booster or low pressure compressor discharge airflow to draw particles out of the flowpath in a manner such as that disclosed in U.S. Pat. No. 4,463,552 entitled "Combined Surge Bleed and Dust Removal System for a Fan-Jet Engine" by Monhardt et. al. The problem with such systems is that amount and force of the bleed flow is dependent on the static pressure of the compressor airflow and is often not strong enough to remove larger pieces and amounts of particles such as ice. Because the bleed flow abruptly curves away from the direction of the compressor flow it is very difficult to hold larger particles in the bleed flow because of their momentum. This problem is common to aircraft, marine, and ground based gas turbine engines. Fan jet engines such as the General Electric CF6-80 series of engines have in series relationship a fan, a booster, and a core engine compressor whereby a portion of the air passing through the fan is ducted to the booster and then the core engine compressor. In order to match the inlet airflow of the core engine compressor to its flight operational requirements and to prevent booster stall a booster variable bleed valve (VBV) is provided in the form of a booster bleed duct having an inlet between the booster and the core engine compressor and an outlet to the fan duct. Opening and closing of the booster bleed duct is conventionally provided by a circumferentially disposed plurality of pivotal doors that retract into the engine structure or casing and are operated by a single unison ring powered by one or more fuel powered actuators. Bellcrank linkages operably connect the retracting pivotal bleed doors to the unison ring. An example of such a stall prevention system using a retracting pivotal door, as compared to a sliding door or valve in the Monhardt patent, is disclosed in U.S. Pat. No. 3,638,428 entitled "Bypass Valve Mechanism" by Shipley et. al. and assigned to the same assignee as the present invention and incorporated herein by reference The operation of the VBV is scheduled by the engine controller, either a mechanical or digital electronic type may be used.
The problem associated with conventional bleed valve ducts and valve doors is that larger particles and amounts of particles such as ice are often not drawn into the bleed duct. The present invention provides the ability to remove larger particles in greater amounts from the compressor airflow in a more efficient manner than has been previously possible
Modern aircraft employ fewer of the higher thrust, fuel efficient, very-high-bypass engines such as the twin engine Boeing 767 aircraft. Aircraft with fewer engines require more total take-off power and more power per engine in order to satisfy the requirement of being able to fly with one engine out. Therefore, the engines are set to lower power settings resulting in less engine airflow during descent when all engines are operational. This results in high water content for engine airflow since the amount of hail or water that gets into the engine is the same so long as the aircraft speed remains the same. On the other hand, higher bypass ratio engines have smaller core flow and larger bullet-nose frontal area. This means more hail or water gets through the compressor into the combustor resulting in higher water content for the air. These two fundamental phenomena combine to cause substantial increase of water-to-air ratio in the combustor resulting in such aircraft engines being more susceptible to engine flame out problem in rain or hail storms.
Furthermore modern high bypass ratio engines incorporating higher pressure core compressors and lower pressure boosters produce less pressure difference between the booster exit and the fan bypass duct. Therefore, it increases the difficulties of bleeding sufficient amounts of air from downstream of the booster to the fan bypass duct for protecting boosters from stall. The current invention fully utilizes the dynamic pressure head to increase bleed capability.
Industrial and marine derivatives of aircraft engines, generically referred to as ground based gas turbine engines or derivative engines, generally replace propulsive elements such as the fan and exhaust nozzle of an aircraft gas turbine engine with a load means such as an electrical generator, ship or marine propeller, or pump (e.g. natural gas pump). Electrical generator gas turbine engines are required to operate at a constant RPM such as 3000 RPM or 3600 RPM in order to generate electricity at a constant 50 Hz or 60 Hz respectively so as to be synchronized with the electrical grid network for which they are used to supply electricity.
Two general types of derivative engines are free turbine and direct drive engines. Free turbine types employ a free power turbine that drives the load and is not directly mechanically coupled to a compressor (usually the booster or low pressure compressor of a multiple rotor engine) of the gas generator used to power the free power turbine. The direct drive turbine is directly mechanically coupled to both a compressor of the gas turbine engine and to the load on a common rotor that is most often the low pressure rotor of a dual rotor gas turbine engine.
A difficult problem is bringing a multiple rotor direct drive engine system to a no-load synchronous speed condition prior to locking in on an electrical grid network which would then hold the low pressure (LP) rotor attached system in a synchronous speed mode. The problem is how to reach zero shaft horsepower (SHP) output with the LP system at synchronous speed without stalling the booster compressor so as to connect and disconnect the gas turbine driven generator from the electrical grid network while not overspeeding the LP system.
The booster stall margin must be controlled by controlling either the inlet flow of the booster or by controlling the booster discharge flow level entering the compressor. Typically booster discharge bleed doors are opened to dump some of the booster flow overboard so as to control the booster operating line to a point below its stall line. Aircraft engine VBV doors normally are inadequate to allow such operation even with state-of-the-art variable inlet guide vane (VIGV) closure on the booster. Current VBV door sizes can be up to 2 times too small to reach this condition.
In adapting an aircraft engine as a derivative for this type of application, since LP speed is held constant to very low powers, a means is needed to minimize required engine changes to accomplish a no-load synchronous speed. For example, it is estimated that for an industrial derivative of the CF6-80C2 engine its VBV bleed doors wide open will only flow 50% of the bleed flow needed to set the booster operating line no higher than its maximum allowable pressure ratio. For example, either the VBV flow area must be doubled or the booster inlet flow must bereduced from 145 lb/sec down to 105 lb/sec.
In accomplishing this, the inlet flow to the core engine compressor must not suffer increased pressure or temperature distortion which could stall the compressor and possibly harm the engine. It is well known that variable inlet guide vanes on boosters and compressors may be used to control the amount of inlet flow and boost pressure at a constant rotor speed by their changing of incidence flow angles to the rotors but it is very difficult if not impossible to avoid stalling the booster or low compressor using this method.